Si Gao

Year of Award

2020

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Forest and Conservation Science

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Thomas H. DeLuca

Commitee Members

Cory C. Cleveland, Benjamin P. Colman, William E. Holben, Anna Sala

Keywords

Biochar, Charcoal, Nutrient cycling, Pyrogenic carbon, Rangeland management, Soil processes

Abstract

Wildfire cause a rapid and sometimes dramatic loss of carbon and nitrogen from forest ecosystems, but it also leaves behind ash and charcoal on the soil surface, both of which affect soil properties, processes, and function. Some of these effects may be induced by applying charcoal or biochar to surface soils. Biochar is the term given to the carbon rich product of thermochemical decomposition of organic material in an oxygen limited environment that is explicitly intended for soil application. Producing biochar from wood residues from timber harvest and applying it to nearby soils may represent a means of reducing carbon emissions associated with wood residue management while providing an innovative approach to potentially improve soil fertility and plant productivity. To date, few biochar studies have been conducted as a part of a holistic closed loop system across ecosystems. The purpose of this dissertation was therefore to improve our understanding of how locally produced wood biochar influences soil nitrogen (N) and phosphorus (P) dynamics in organic agriculture, temperate forest, and semi-natural rangeland ecosystems in the Northwestern US. Several key findings from the experiments conducted at sites in WA and MT include: (1) Applying wood biochar alone on a relatively fertile agricultural soil generally had a neutral effect on soil N turnover, but by contrast, biochar increased soil nitrification and N mobility in a natural, organic rich rangeland ecosystem; (2) Combining wood biochar with an organic fertilizer created positive synergistic effects on soil N cycling rates and availability while reducing N leaching potential; (3) Soil P bioavailability was generally increased by wood biochar application regardless of ecosystem type or the combined use of fertilizer. This result appeared to be primarily a function of biochar characteristics and potentially associated with abiotic P mobilization processes rather than biotic mechanisms; (4) Slight acidic soils benefit from wood biochar the most at their multi-functionality in N or P cycling compared to pH neutral or alkaline soils; (5) Wood biochar immediately accelerated solution N flux rates in the charosphere of temperate mixed-forest soil that features a sandy loam texture and neutral pH, a result highlighting the uncertainty in, and the dynamism of, the responses of nutrient pools and fluxes to biochar additions across different scales; and (6) Wood biochar did not impart any negative impacts on soil processes examined in these studies. Overall, this work provides an important contribution to our collective knowledge of the value and function of locally produced wood biochar as a bio-enhancing soil amendment for ecosystem nutrient management in the Northwestern US.

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